Most of the fabrication processes for semiconductor devices are conducted in a semi-closed fabrication chamber with the continuous flow of one or more gaseous components therethrough via a vacuum pump, which is typically a turbo pump. These processes include plasma etching, reactant ion etching, and various modes of chemical vapor deposition, etc. To facilitate the even distributions of the flowing gas, a baffle plate, or exhaust plate, is horizontally installed inside the fabrication chamber, dividing the fabrication chamber into an upper chamber and a lower chamber.
FIG. 1 shows an illustrative schematic cross-sectional view of a conventional semiconductor fabrication chamber used in reactive ion etching (RIE) processes. The chamber 11 is divided by a horizontally disposed baffle plate (or exhaust plate) 12 into an upper chamber 13 and a lower chamber 14. The fabrication chamber 11 also includes a cathode 15, a focusing ring 16, and an electrostatic chuck (ESC) 17 upon which a wafer 18 is disposed. The fabrication chamber 11 is enclosed by a chamber wall 19 and a chamber lid 20. An insulating ring 21 is placed between the baffle plate 12 and the cathode 15.
FIG. 1 also shows that a turbo pump 22 and a throttle valve 23 are used to pump one or more gases into the fabrication chamber 11. FIG. 2 is a top view of a conventional baffle plate 12. Typically, the baffle plate 12 is a ring-shaped plate having a plurality of radially extending perforations 24. The baffle plate 12 is typically separated from the cathode 15 by an insulating ring 21.
To control the pressure inside the fabrication chamber and the flow of gas into the chamber, a pressure gauge 25 is often installed. The pressure gauge 25 is connected to the throttle valve 23 via a CPU 26, as shown in FIG. 3. Analog signals obtained from the pressure gauge 25 are converted into digital signals by a D/A converter, which are then compared against a predetermined value stored in the CPU. The CPU then sends control signals to the throttle valve so that appropriate adjustments can be made to maintain the gas pressure inside the fabrication chamber at or close to the predetermined value. During plasma-related fabrication processes, such as plasma etching, plasma-enhanced chemical vapor deposition, etc., because plasma, or ionized gas, can cause instability in the pressure measurement, the pressure gauge is typically installed in the lower chamber, to avoid interference from the plasma gas.
It was discovered by the inventor of the present invention that, after repeated usage of the fabrication chamber, certain particulate materials, such as polymers that are formed during a plasma etching process, can accumulate on the baffle plate, thus reducing the openings of the baffle plate through which reactant gas enters. As a result, the pressure gauge, which is located in the lower chamber, may not accurately tell the true pressure in the upper chamber. This resulted in inadequate and improper pressure control inside the fabrication chamber. In many cases, this problem can explain the reduced fabrication yield rates. This problem, which becomes more noticeable and serious as the fabrication of semiconductor devices moves into sub-micron age, which requires more precise control of the gas pressure inside the fabrication chamber, also becomes more profound with extended usage of the fabrication chamber.
As semiconductor devices are becoming more like a common commodity, the profit margin of fabricating semiconductor devices is constantly decreasing. As a result, it is important to look at every process parameter that may affect the failure or rejection rate of the fabricated products, so as to reduce production cost. Since imprecise pressure control inside the fabrication chamber can have a significant impact on the product rejection rate, it is important that the above-mentioned problem be carefully studied at and that a suitable solution be developed so as to improve the product yield rate.